In the past, vehicle locating systems have been provided which utilize conventional compass devices for calculating an on-board location of a vehicle. While those "dead-reckoning" systems have been generally satisfactory, they are subject to several limitations. For example, when such systems are utilized in applications involving a severe environment in which physical impact, weather, and various other adverse factors are present, the conventional compass device provides a source of error. Further, such systems are also subject to the accumulation of errors unless they are periodically and automatically updated. U.S. Pat. Nos. 3,749,893 and 3,789,198 disclose exemplary dead reckoning systems and are included herein by reference.
Some proximity prior art systems do not even attempt to trace the incremental movements of the vehicle. In such systems, transceivers are placed at predetermined locations within an area and it can only be determined that a vehicle is within a designated range of those predetermined locations. In many applications, the precision of such a system is inadequate. Further, to improve the precision of those systems by increasing the number of transceivers and predetermined locations, is generally prohibitively expensive. U.S. Pat. No. 3,772,691 shows such a proximity system and is included herein by reference for its teaching of a proximity-type system.
Thus, there is a need for an improved vehicle locating system utilizing a solid state compass suitable for operation in an adverse environment and having the advantages of both dead reckoning and proximity location fixing techniques.